The Primal and Future Moon

Most of us
appreciate the beauty of a full Moon sedately moving across our night
skies. The Moon seems unchanging, as if for eons it has been there,
controlling the tides, going through its phases every month and
inspiring poets and lovers. It might surprise you to know that in
the past, things were much different, and they are still changing.

A
billion years ago, our relationship with the Moon was quite
different. For one thing, the Moon was much closer and therefore
appeared much larger: and we saw the entire Moon, not just one face
as we do now. It took the Moon only twenty days to circle the Earth,
and Earth's day was only eighteen hours long. Massive tides, over a
kilometer in height, would ebb and flow every few hours. From the
beginning though, things were changing, as the Moon's orbit was
getting larger.

Every
year, the Moon moves about four centimeters (about 1.6 inches,)
farther out into space, away from Earth. Right now, the Moon's
rotation rate, and the time it takes to orbit Earth are the same. Is
this all just some cosmic coincidence? Would it surprise you to know
that at one time, Earth didn't even have a moon? Where then, did our
nearest neighbor in space come from, and what will the future bring?
To answer these questions we must go very far back into time.

Four and a
half billion years ago the Earth was about sixty percent formed,
though it did have a differentiated core, crust and mantle. In these
primal skies, there was no Moon. It was a very violent time in the
Solar System with other planets forming from clumps of gas, dust and
rock and impacts among these bodies were the norm. Orbits were not
the sedate, orderly system they are today.

One
planet, about the size of Mars, probably had an orbit which crosses
Earth's, and eventually a collision occurred. The impacting planet
made a hard, but glancing blow off Earth at just the right angle. It
almost bounced off Earth, but was consumed instead. But the blow sent
shock waves across Earth, spewing gas and debris into space, giving
Earth, for a very short time, a ring around it.

The debris
around Earth began to condense into clumps and quite rapidly, a blink
in cosmic time, formed a large glowing ball: our primal Moon. The
Moon would have looked about ten times larger than it does today, and
the Earth was changed forever.

This story,
about the birth of the Moon, is still a theory, one among several
attempting to explain the source of our satellite. Currently, this is
the most widely accepted of the theories.

Since
that cosmic collision, the Earth and Moon have both have an effect on
each other, and will continue to have an affect far into the future.
Since the Earth is so much more massive than the Moon, the Earth has
had the larger effect. Earth's gravity has caused the Moon to become
tidally locked us as well as increasing the distance between the two
worlds. By tidally locked, we mean that the moon's rotation rate is
the same as the time it takes the moon to go once around Earth, which
also results in us seeing only one side of the Moon.

The
dynamics of this Earth/Moon relationship come basically down to
Earth's tides so we need to take a closer look at them. We know that
the Moon is the primary cause of Earth's ocean tides by the Moon
“pulling” on the Earth as it goes around it. But gravity lessens
with distance and this means that the pull from the Moon is stronger
on the side of Earth that is facing the Moon.

Keep
in mind that the Moon is not pulling on the Earth as a whole, but
rather on the part of Earth beneath the Moon. So, as the Moon passes
over a part of Earth, the Moon pulls on that area. If it is over
water, the Moon actually pulls on the water, creating a void for more
water to flow into that area and creating a high tide. Directly
opposite that spot, the Moon's gravity is pulling on the Earth
itself, pulling it away from the water and allowing more water to
flow into that area creating another high tide. But because
the distance from the Moon is greater, the pull from gravity on the
far side of the Earth is about six percent less than on the side
facing the Moon.

The Sun
also has an effect on tides, but it only about forty six percent of
the tidal effect from the Moon. That might sound confusing – seeing
as how the Sun is so much bigger than the Moon, but remember –
gravity decreases with distance – and the Sun is much farther away
from Earth than the Moon.

When the
Sun and Moon team up together (both on the same side of the Earth) we
have even larger high tides. The point though, is that Earth is being
tugged from several bodies, rotating under the shifting tides.

Also,
remember that as the Moon is moving around the Earth, the Earth
itself is spinning. The Earth spins much faster than the Moon moves
around our planet. The effect this has, is that that the tidal bulge
caused by the Moon is actually pulled ahead of the Moon by the
Earth's faster rotation.

The Moon
therefore is pulling back on the tidal bulge with the result that
there is friction between the ocean floor and the water. The friction
actually slows Earth down, therefore explaining why our days on Earth
are getting longer.

These tidal
bulges on Earth, have an affect on the Moon, in essence pulling the
Moon and forcing it into a higher orbit. This is why the Moon is
moving away from us.

We
mentioned tides on Earth caused by the Moon – but Earth actually
causes tides on the Moon's surface as well. Even though the Moon has
a solid surface of rock, small tidal bulges actually occur. Earth
tugging on the Moon's surface has slowed down the rotation of the
Moon over time. Over time the Moon rotation slowed until it equaled
the time it took the Moon to go around the Earth. At that point,
those lunar tidal bulges lined up with Earth and the rotation ceased
slowing down: the Moon was tidally locked with Earth and that's why
we see only one side of the Moon.

Just
because the Moon's rotation has ceased to slow down doesn't mean that
the Earth/Moon system isn't changing in other ways. We already know
that the Moon is moving farther away from Earth but other changes are
happening as well.

We
mentioned that Earth's rotation was being slowed by the friction
between the oceans and the ocean floor. This will continue to happen
until Earth's tidal bulges align with an imaginary line running
through the center of the Earth/Moon system then Earth's rotation
will cease slowing down. This will take a few billion years but when
it does happen: Earth's day will be a month long (960 hours a day)
and our month will be forth days long! By then the Moon will be
twenty-five percent farther away. If we were on the Moon looking back
at Earth – we would see the same face of Earth – just as now we
see only one face of the Moon. And if someone were still on Earth:
the Moon will have moved far enough away that it appears much smaller
– and there are no more solar eclipses!

We have yet
to answer that question about a cosmic coincidence. Do we “just
happen” to be here on Earth when the Moon is close enough to give
us tides, and slowed the Earth's rotation until we have a twenty-four
hour day and thirty day month? Some scientists believe that it may
be thanks to the Moon that we are here at all.

Although
the Moon is slowing down Earth's rotation, it has stabilized it as
well. If we had no moon, we would suffer movement of the Earth's
poles, an axial tilt of up to ninety degrees, causing major climate
swings. Some scientists believe that under those circumstances:
intelligent life simply could not have evolved.

It is hard
to imagine that our world as we know it, and all that is familiar to
us, would likely not exist if not for the Moon. We may truly owe our
lives to a lunar legacy.